It is understood that, in mechanical systems, in the case of two components which are movable relative to each other under friction, a frictional force directed counter to the direction of motion occurs when the components are moved relative to each other. In many mechanical systems, this frictional force may act interferingly, since it may prevent a clear stationary state from setting in. In addition, it leads to hysteresis in response to reversing motional sequences and to a so-called “stick-slip behavior”.
From this it is also known that, for the reduction or even prevention of these effects, the material pairings and/or the surface treatments of the two components (“friction partners”) may be matched so that their coefficient of static friction and coefficient of sliding friction is minimized. Appropriate lubricants are also used, in order to reduce the frictional force. Lastly, it is also known that one may generate a micromotion modulated upon the base motion of a friction partner, in order to maintain the mechanical systems, according to the so-called “Stribeck curve”, durably in sliding friction, instead of intermittently in sliding friction and in static friction. Such an oscillating micromotion is called “dither”. The dither known up to now is symmetrical, that is, the two half-waves within a period of oscillation do have opposite signs, but are otherwise identical. Such a symmetrical, and thus average-free dither is used, for instance, in controlling an electromagnetically operated proportioning valve, which is used in automatic transmissions of motor vehicles. It is ensured thereby that the transmission characteristic pressure/current is not corrupted and that the pressure/current hysteresis is reduced.
German document DE 103 15 152 A1 discusses a method for controlling an electromagnetically operated valve using a pulse width modulated signal whose period duration depends upon the valve current or an “on” portion of the control signal. German patent document DE 103 04 711 B4 discusses a similar method, in which the pulse frequency of the control signal is a function of a valve current and/or a supply voltage of the electromagnetic valve.